Image display devices using organic electroluminescence (EL) elements are known as image display devices using current-driven light-emitting elements. Such organic EL display devices using organic EL elements which emit light are best suited to make thinner devices because such organic EL elements eliminate the necessity of back lights conventionally required for liquid crystal display devices. In addition, the organic EL elements do not place a limit on view angle, and thus are expected to be practically used as next-generation display devices. Furthermore, the organic EL elements used for the organic EL display devices include light-emitting elements whose luminance are controlled by currents having certain values, instead of including liquid crystal cells controlled by voltages applied thereto.
In a usual organic EL display device, organic EL elements which serve as pixels are arranged in a matrix. An organic EL display is called a passive-matrix organic EL display, in which organic EL elements are provided at intersections of row electrodes (scanning lines) and column electrodes (data lines) and voltages corresponding to data signals are applied between selected row electrodes and the column electrodes to drive the organic EL elements.
On the other hand, an organic EL display device is called an active-matrix organic EL display, in which (i) switching thin film transistors (TFTs) are provided at the intersections of scanning lines and data lines and connected with the gates of driving elements which receive data signals through the signal lines when the switching TFTs are turned on through selected scanning lines, and (ii) the organic EL elements are driven by the driving elements.
The organic EL elements, which are included in the passive-matrix organic EL display device and which are connected to selected row electrodes (scanning lines), emit light only until the selected row electrodes become unselected. In contrast, organic EL elements in the active-matrix organic EL display device can keep emitting light till subsequent scanning (or selection), by including driving TFTs for controlling current supplied to the organic EL elements by using voltages applied to the gate electrodes, and electrostatic capacitors for stably holding the gate voltages of the driving TFTs. Thus, there is no reduction in luminance of the display even when the number of scanning lines increases. Accordingly, the active-matrix organic EL display device is driven with a low voltage, thereby consuming less power.
Here, an application of the gate voltage causes stress on the driving TFT, which changes the state of the driving TFT into a stable state slightly different from the initial electrical characteristics (threshold voltage). More specifically, in a case where display patterns are different between a previous display period and a subsequent display period, different levels of voltages are applied to the gate voltage of the driving TFT; and thus, the stable state of the electrical characteristics of the driving TFT in the previous display period caused by the application of gate voltage is different from the stable state of the electrical characteristics of the driving TFT in the subsequent display period caused by the application of gate voltage different from the gate voltage applied in the previous display period. This causes display unevenness (a residual image) in which influence of the previous display period is displayed at the moment of switching from the previous display period into the subsequent display period, resulting in reduction in display quality.
In order to cope with this, for example, Patent Literature (PTL) 1 discloses a circuit configuration of pixel units in an active-matrix organic EL display device.
FIG. 15 is a circuit configuration diagram of a pixel unit in a conventional organic EL display device according to PTL1. A pixel unit 500 in FIG. 15 has a simple circuitry including: an organic EL element 505 having a cathode connected to a negative power source line (whose voltage value is denoted as VEE); an n-type thin film transistor (n-type TFT) 504 having a drain connected to a positive power source line (whose voltage value is denoted as VDD) and a source connected to an anode of the organic EL element 505; a capacitor element 503 which is connected between the gate and the source of the n-type TFT 504 and holds a gate voltage of the n-type TFT 504; a third switching element 509 for causing both the terminals of the organic EL element 505 to have approximately the same potential; a first switching element 501 which selectively applies a video signal from a signal line 506 to the gate of the n-type TFT 504; and a second switching element 502 for initializing (resetting) the gate potential of the n-type TFT 504 into a predetermined potential. The following describes light emitting operations performed by the pixel unit 500.
In this conventional technique, in order to initialize (reset) the n-type TFT 504, first, the second switching element 502 is turned on at the start of a frame period by a scanning signal supplied from the second scanning line 508, and a predetermined voltage VREF supplied from a reference power source line is applied to the gate of the n-type TFT 504 so as to prevent a current from flowing between the source and the drain of the n-type TFT 504.
Next, the second switching element 502 is turned off by a scanning signal supplied from the second scanning line 508.
Next, the first switching element 501 is turned on to apply a signal voltage supplied from the signal line 506 to the gate of the n-type TFT 504.
Next, the third switching element 509 is turned off to supply a signal current corresponding to the charge accumulated in the capacitor element 503 from the n-type TFT 504 to the organic EL element 505. At this time, the organic EL element 505 emits light.